//sv_vote_gametype_mygametype_name "My Mode" // Name displayed for the custom gametype "mygametype" (replace "mygametype" with the name you set in sv_vote_gametype_options)
//sv_vote_gametype_mygametype_type "ft" // the basetype that the custom gametype is derived from
//sv_vote_gametype_mygametype_description "This is my custom gametype" // the description for the custom gametype
//// Custom icons for custom gametypes are supported and have to be located in a server pk3 as gfx/menu/default/gametype_mygametype

Duel shows up in the list, but doesn't appear to work correctly (it plays as a straight deathmatch game). I've commented it out just in case I've broken something by overwriting it. The instagib cvars just return an error when they're loaded:

Though there is a second weird error that shows up when the vote screen is rendered:

That sorta dovetails into my next question (which is, what are the standard settings for Duel).
So, no powerups, playerlimit 2. Is there anything else (I vaguely remember something about non mega health and armours not being able to take HP above 100?)

Not being able to take 25 and 50 health/armor above 100HP/Armor is part of XPM (Xonotic Pro Mode). XPM is used in some modes like duel and competitive TDM. You can look at https://github.com/packer-/server-configs to see how packer handles it (see pickup-extra.cfg)